Menuflyout fix

[cvirendra]

Currently when number of MenuFlyoutItem change, they don't reflect in the MenuFlyout as expected. For example, if you have a MenuFlyout that looks like this:

Then something changes that removes Feedback on recommendation to be removed, the MenuFlyout appears like this:

instead of like this:

This PR fixes this issue.

Microsoft Reviewers: Open in CodeFlow

[jonthysell]

@microsoft-github-policy-service rerun